P2: Electrical Circuits

Charging objects A balloon rubbed on a sleeve will gain electrons from the sleeve and become negatively charged. The sleeve becomes positively charged since it loses electrons Qu: If the sleeve has gained positive charges, how many electrons have been gained by the balloon? Rubbing your feet on a nylon rich carpet and then touching a radiator will result in a shock. How?

Electrostatic Forces A gold leaf electroscope uses this principle to measure the amount of charge. The more charge the further the gold moves away from the metal stem Qus: 1.Why does the girl’s hair (opposite) stand on end? 2.Why does a balloon rubbed on your sleeve pick up smalll pieces of paper?

Current: A flow of charges Electrons in wires (negatively charged) Ions in solutions (positive and negatively charged) measured in amperes (A) Found using an ammeter connected in series Charges are always present but they need a battery OR mains to push them along Scientists making life complicated? Electrons flow from the negative electrode (terminal) of the battery round the circuit to the positive terminal… however, historically scientists thought the charges were positively charged so the current is often shown flowing the other Way [CONVENTIONAL CURRENT!] Conventional current Electron flow

Is current used up? NO! But the chemical energy stored in the battery is as it is transferred to the components/ appliances. Series circuits: The current is the same at all points If one component breaks then the charges stop flowing so no current Found in some Christmas lights (much to your parents annoyance if one bulb is faulty!) Parallel circuits: The flow of charge splits at the branches flowing away from the battery and rejoins flowing towards the battery If one component breaks then the components in the other branches carry on working

How does the electrical energy get reduced? Every appliance has a power rating showing how many joules per second it transfers (measured in watts) The electrical energy transferred to the appliance will be either useful (e.g. light for a lamp) OR wasted (e.g. heat for a lamp). The more useful energy the more efficient The longer the appliance is on for the more energy is transferred and so the more it costs. Energy Transferred = (kWh) Power rating X (kW) Time used (h) The electricity board set a cost per kWh used… This is shown by your electricity meter. efficiency % = useful energy/ total energy X 100

Changing the current… By changing the voltage… The bigger the voltage of the battery/ supply the bigger the current (since each charge gets more energy… a bigger push!) Voltage is measured in volts (V) Measured using a voltmeter By changing the resistance… The bigger the resistance the smaller the current (since the flow of charges are opposed… a bit like friction!) Resistance is measured in ohms (Ω) Calculated from current and voltage using…

Ohm’s Law Current is proportional to the voltage for resistors The resistance is given by the gradient Current (A) Calculate the resistance above using the Ohm’s Law Equation Voltage (A)

Changing the resistance… Variable resistors controls resistance using a slider OR a dial Thermistors Resistance decreases as it gets hotter Useful for thermostats Light dependent resistors (LDR) Resistance decreases as it gets lighter Useful for the automatic switching on/off of lights during the day/ night

Resistors in circuits Series –Just add the resistance of each Calculate the total resistance opposite… Parallel –Resistance will be less than the individual resistors –For the boffins… 1/R=1/R1+1/R2 Calculate the total resistance opposite…

Potential difference We can still measure a voltage across a component like a lamp even though a lamp doesn’t “push” the charges… this is the potential difference across the component. Series parallel

Power We can increase the power (work done per second/ watts) by… Increasing the potential difference (VαP) Increasing the current (IαP) To calculate the power we can use… P=V x I However we often rearrange this to find the current e.g. for a suitable fuse… P/V = I The power of an appliance is normally written on the back and the voltage in the UK is normally 230V for mains. Penguins visit Iceland

Finding the correct fuse Fuses can be found in lots of different ratings but 3A and 13A are the most common Fuses have a low melting point wire which melts when too large a current flows Brass cap Low melting point wire Ceramic tube A wii console has a power rating of 36W and uses a 12V supply. Calculate the current and then explain why the two fuses mentioned are unacceptable Car fuses

Generating electricity If a wire (or even better a coil of wire) cuts the field lines of a magnet a potential difference is formed… the size of which is determined by The strength of the magnet The number of turns on the coil The speed of the movement The core in the coil; soft iron is best process: electromagnetic induction note how the voltmeter needle goes from side to side…. An alternating current (a.c.)

Transformers These do not need moving parts to induce a potential difference A coil with an AC supply will induce another AC provided the two are joined via a soft iron core You can step up the voltage OR step down the voltage BUT not alter the power. Voltage on secondary = number of turns on secondary Voltage on primary number of turns on primary Vs = Ns Vp If a primary coil has 100 turns and the secondary has 1000 turns what will be the secondary voltage if the primary voltage is 20V?

National Grid Uses transformers to minimise energy loss by keeping voltages high but current very low so there is less heating of the wires on the way to our homes Electricity generated at 25000V is stepped up to V for distribution Local substations step down the voltage to 33000V for industry and 230V for homes